In a digitally controlled printing system, a receiver media (also called a print media) is directed through a series of components for printing an image. The receiver media can be a continuous web of media or a sequential flow of cut sheets of media. In the case of a cut-sheet printing system, a media transport system physically moves the receiver media sheets through the printing system. As the receiver media sheets move through the printing system, a printing process is carried out on a first side of the receiver media sheets. For example, in an inkjet printing system, liquid (e.g., ink) is applied to the receiver media sheet by one or more printheads through a process commonly referred to as jetting of the liquid.
In many printing applications it is desirable to print on both sides of the receiver media sheets, thereby saving cost and being more environmentally friendly. Some printing systems are capable only of printing on a single side of the receiver media sheets. In this case, a user who wishes to print on both sides of the receiver media sheets can print the odd numbered pages, reload the stack of print media sheets, and then print the even numbered pages. However, this is slow and cumbersome. A more user-friendly printing system is one that includes a media inverter, also called a duplexer, for duplex printing.
Desktop printing systems typically use a carriage to move a printhead across the receiver media sheet to print a swath of an image and advance the receiver media sheet between swaths in order to form the image swath-by-swath. Such printing systems are small and low-cost, but printing throughput on single sides of letter-sized receiver media sheets is typically limited to around 20-30 pages per minute. Because the distance the receiver media sheet is moved through a desktop printing system is small, the transport system can be a series of rollers. Printing of all of the colors of the image is performed in a relatively small print zone compared to the length of the receiver media sheet. For printing a single side, the receiver media sheet is advanced swath-by-swath sequentially past the print zone. For duplex printing, the receiver media sheet is typically driven through a duplexer by one or more rollers to turn the receiver media sheet over and return the receiver media sheet to a point prior to the print zone so that the second side can be printed.
High-volume cut-sheet printing systems typically print one color of an entire line of the image essentially all at once, for example using a page-width printhead or some other page-width printing process in a printing station for that color. The receiver media sheet is advanced past the printing station as sequential page-width lines of the same color are printed. To print all colors (typically cyan, magenta, yellow and black), the receiver media sheet is moved from printing station to printing station, each printing station printing a different color. In a high volume inkjet printing system, there are typically dryers between some or all of the printing stations in order to remove some of the carrier fluid of the ink and make the ink less mobile so that it is less susceptible to bleeding into the next color that is printed.
In web printing systems, tension in the continuous web of receiver media can be used to pull the web through the various printing stations. In high-volume cut-sheet printing systems, a media transport system, which typically includes components such as belts or drums, is used to move the receiver media sheets through the printing system from one printing station to the next. High-volume cut-sheet printing systems tend to be significantly larger and more costly than desktop printing systems. However, the printing throughput is also typically significantly higher.
Because of the successive printing stations, and other stations such as dryers or fusers, in a high-volume cut-sheet printing system, the distance between the input to the first printing station and the output of the last printing station can be relatively large compared to the length of the receiver media sheet. A simple roller-driven duplexer that can position the lead edge of the receiver media sheet close enough to the print zone that a feed roller can begin to pull the leading edge before trailing edge of the receiver media sheet passes the duplexer drive roller is not adequate in such a large high-volume cut-sheet printing system. Furthermore, some high-volume cut-sheet printing systems include a first printing module including all of the color printing stations for printing a first side of the media sheets, and a second printing module including all of the color printing stations for printing a second side of the media sheets. A media inverter is positioned between first printing module and the second printing module.
Many cut sheet media inverters that are disclosed in the prior art include one or more rollers or other rotatable member(s) that reverse direction as part of the media inversion process. U.S. Pat. No. 5,374,049 to Bares et al., entitled “Compact inverter,” discloses a reversible roller onto which a sheet is scrolled and subsequently unscrolled, thereby reversing the lead and trail edges of the sheet.
U.S. Pat. No. 6,851,672 to Shmaiser, entitled “Sheet transport position and jam monitor,” includes a “perfector transporter” that changes direction of rotation from counterclockwise to clockwise during inversion of the sheet.
U.S. Pat. No. 4,496,142 to Iwasaki et al., entitled “Sheet supplying device for two-sided copying,” discloses holding one-side-copied sheets by a plastic film wound on a scroll in a first direction, and then reversing the scroll to invert the one-side-copied sheets.
U.S. Pat. No. 7,400,855 to Bokelman et al., entitled “Winding media,” discloses winding sheets of paper around two winding members that wind the paper beginning in the middle of the paper rather than at an end. The winding device reverses direction to enable unwinding the paper for inversion.
U.S. Pat. No. 6,463,256 to Blackman, entitled “Duplexing module for printer,” discloses a feed roller that reverses direction during the media inversion process.
U.S. Pat. No. 5,772,343 to Beretta et al., entitled “Media handling system for duplex printing,” discloses winding the paper on a metering roller. The direction of the metering roller is reversed during sheet inversion.
U.S. Pat. No. 5,772,343 to Beretta et al., entitled “Media handling system for duplex printing,” discloses a first and second roller. During first side printing, a media sheet is fed along the first roller and directed to a second roller. After a drying time, the second roller reverses direction moving the media sheet back toward a reefed guide.
U.S. Pat. No. 6,241,236 to Bokelman, entitled “Automated sheet delivery to selected paths using reversible crenellated roller,” discloses a transport roller assembly having a direction of rotation that is reversed to re-route the sheet back for further processing.
Since high-volume cut-sheet printers have capability for high printing throughput, other components of such a printing system should be able to keep up with the printing throughput so that they do not compromise the overall throughput of the system. A media inverter that reverses direction of a roller or other rotatable member will have a limitation in throughput that is related to the slowing down, stopping and reversal of the roller direction. In addition, it is desirable that high-volume cut-sheet printing systems not be excessively large. There is an ongoing need for a media inverter that is compact and high speed for turning the cut receiver media sheets over for printing the second side of the media sheet.